Mammoth Cave National Park

In the late 18th century, white settlers discovered an abundance of saltpeter, an ingredient of gunpowder in this cave system, which is the most extensive series of natural passageways in the world. The cave became an important source of saltpeter during the War of 1812. Later it drew tourists and tuberculosis patients, the latter hoping for a cure in the cave's allegedly regenerative air. Two-hour historic cave tours travel 300 feet underground through the narrow passageway Fat Man's Misery, up Mammoth Dome, and across a bridge spanning the 105-foot-deep Bottomless Pit. The surrounding 52,800-acre park features scenic bluffs, springs, and historic attractions, such as the train engine No. 4 and its coach from the railroad during the late 1800s.